DEVELOPMENT OF THE THEORY OF ISOSTASY 289 



earth's crust being in a condition analogous to hydrostatic equilibrium, 

 saying : 



"I conceive that there can be no other support Lfor an elevated tableland) 

 than that arising from the downward projection of the earth's light crust into 

 the dense lava ; the horizontal extent of that projection corresponding rudely 

 with the horizontal extent of the tableland, and the depth of its projection 

 downwards being such that the increased power of flotation thus gained is 

 roughly equal to the increase of weight above from the prominence of the 

 tableland. . . . This fluidity [of the interior of the earth] may be very 

 imperfect ; it may be mere viscidity ; it may even be little more than that de- 

 gree of yielding which (as is well known to miners) shows itself by changes 

 in the floors of subterraneous chambers at a great depth when their width 

 exceeds 20 or 30 feet; and this yielding may be sufficient for my present ex- 

 planation. . . . The diminution of attractive matter below, produced by the 

 substitution of light crust for heavy lava, will be sensibly equal to the increase 

 of attractive matter above. In all the latter inferences it is supposed that the 

 crust is floating in a state of equilibrium. But in our entire ignorance of the 

 modus operandi of the forces which have raised submarine strata to the tops 

 of high mountains, we can not insist on this as absolutely true. We know 

 (from the reasoning above) that it will be so to the limits of breakage of the 

 tablelands ; but within those limits there may be some range of conditions 

 either way." 



The idea of isostasy was here distinctly expressed by Airy, but with an 

 arrangement of compensation now considered improbable. 



In a paper 3 on this subject in 1859 Pratt agreed that the compensating 

 cause must be looked for in deficiency of matter below the mountain mass, 

 but proposed a different hypothesis : 



"At the time when the earth had just ceased to be wholly fluid, the form 

 must have been a perfect spheroid, with no mountains and valleys nor ocean- 

 hollows. As the crust formed, and grew continually thicker, contractions and 

 expansions may have taken place in any of its parts, so as to depress and 

 elevate the corresponding portions of the surface. If these changes took place 

 chiefly in a vertical direction, then at any epoch a vertical line drawn down 

 to a sufficient depth from any place in the surface will pass through a mass 

 of matter which has remained the same in amount all through the changes. 

 By the process of expansion the mountains have been forced up, and the mass 

 thus raised above the level has produced a corresponding attenuation of matter 

 below. This attenuation is most likely very trifling, as it probably exists 

 through a great depth." 



In 1865 a series of pendulum observations of the force of gravity was 

 made in India, some at high altitudes, and from these conclusions were 

 drawn, first, that the force of gravity "is very much less on the summit 

 of the highly elevated Himalayan tablelands than can be accounted for 



3 Phil. Trans. Royal Society, vol. 149, p. 747. 

 XIX— Bull. Geol. Soc. Am., Vol. 33, 1921 



